The present invention relates to a process for regulating centrifuges which are used for solid/liquid separation in particular for the dewatering of sludges.
It is known that the purpose of a centrifuge, in its application to the dewatering of sludges, is to ensure solid/liquid separation of the incoming effluent (or sludge) so as to obtain:
on the one hand a cake or sediment of pasty consistency;
on the other hand, a liquid laden with little suspended matter (SM).
In order to facilitate good separation between the solid phase and the liquid phase, and to promote the capture of the solid particles by the centrifuge, a reagent (polymer) is added to the sludge.
The conventional techniques for dewatering by centrifuging are not generally optimal with regard to the following four criteria:
the stability of operation of the centrifuge;
the solids content of the filter cake;
the permanent control of the capture rate and,
the dosing of the polymer.
Experience shows therefore that the implementation of centrifuges requires regulation so as to maintain the centrifuge in the best operating zone despite the variations in concentration and in quality of the incoming sludge, whilst optimizing the dose of reagent injected and minimizing the quantity of suspended matter which is not picked up by the centrifuge and which is found in the centrate.
When designing the plant, it will also be necessary to choose between conventional centrifuges and so-called xe2x80x9cintensivexe2x80x9d centrifuges with a high degree of fill of dry solid matter.
The conventional regulating processes involve measuring the concentration of suspended matter and they may comprise:
1) regulation of the mass flux entering the centrifuge through measurement of the concentration and of the hydraulic discharge acting on the discharge from the booster pump. The main problem to be solved is the reliability of the in-line SM sensor depending on the type of sludge: the response of this sensor is limited in terms of concentration influenced by strong colouring agents and disturbed by tows. These limits reduce the field of application of this regulation to a few particular cases, all the more so since the said regulation cannot take into account variations in quality of sludges as a function of their origins and of their relative proportions (fresh sludges, primary settled sludges, digested sludges etc).
2) Feedback control of the dose of reagent proportionally to the flux entering the centrifuge whether or not the regulation of the mass flux is operational. It is thus possible to envisage more economical management of the polymer dosing.
3) Feedback control of the dose of reagent to the hydraulic discharge entering the centrifuge. This is a particular case of the feedback control mentioned in paragraph 2 above which regards the concentration of the incoming effluent as being xe2x80x9cconstantxe2x80x9d. In fact, there must be an excess dosage of reagent so as to offset the inevitable variations in concentration.
4) Regulation of the dose of reagent based on measuring turbidity in the clarified effluent (centrate) by implementing an in-line SM concentration sensor on the drive. The objective of this measurement is to influence the coefficient of proportionality to the reagent dosage flow rate by way of a regulator. In fact, the centrate is not well suited to in-line SM concentration measurement, the latter being disturbed in particular by formations of foams, microbubbles, etc.
Thus for example, WO 97/20 634 describes a process as well as a device for operating and controlling a continuous-feed centrifuge which consist in measuring in real-time in particular the flow rate of sludges and/or of reagents, the suspended matter content, the value of the torque of the centrifuge so as to adjust in particular the flow rate of sludges entering the centrifuge.
Experience shows that these conventional modes of regulation (or the absence of regulation) induce critical and unstable operation of centrifuges, with regard to target values, thus demanding the presence of staff in order to make adjustments and obtain correct operational performance. It will be recalled that this performance is essentially characterized by:
satisfactory solids content of the outgoing sludge;
clarified effluent (centrate) which is sufficiently clear;
a reasonable dose of reagent (polymer).
The purpose of the present invention is to guarantee the abovementioned performance without employing monitoring staff, that is to say to obtain the following characteristics automatically:
optimal solids content of the outgoing sludge without excess polymer;
optimal mass flux irrespective of the variations in the concentration of the sludge entering the centrifuge and,
optimally clarified effluent (with no return of pollution to the head of the sewage station).
The applicant is moreover the proprietor of FR-A-2 707 758 which relates to a device for continuously measuring the concentration of suspended matter of a centrate, this device making it possible to carry out a reliable and continuous measurement of the suspended matter content of the liquid phase, the so-called xe2x80x9ccentratexe2x80x9d.
The present invention is characterized by the fact that the regulating of the centrifuge is carried out via fuzzy logic using the signal from the sensor according to FR-A-2 707 758 as well as the other signals available on the centrifuge, thereby making it possible to control the flow rates of sludge and of reagent supplied to the said centrifuge.
A simplified explanation of fuzzy logic will be given hereinbelow.